The morphology of the surfaces of the cells of normal and T/t locus mutant mouse embryos, and of sperm from normal and T/t locus mutants, is under investigation in the scanning electron microscope. Correlated transmission electron microscopy is also employed to amplify and aid in the correct interpretation of scanning electron images. Mutant genes at the T/t locus have the unique property of impairing both embryonic development and the production and function of sperm. Moreover, serological techniques have demonstrated cell surface antigens, determined by genes at the T/t locus on sperm and early embryos, thus establishing the long-held contention that the T/t locus genes specify cell surface properties of sperm and embryos and substantiating the interpretation that mutations at this locus interfere with normal processes of embryonic and cellular differentiation by altering cell surface properties. Therefore, we intend to employ the methods of immuno-electron microscopy and fluorescence microscopy to determine the topography of T/t locus antigens on embryos and male germ cells and to explore the role of T/t locus antigens on the specific groups of cells during embryonic development and during sperm differentiation.